Deposits of oil and natural gas beneath the surface of the earth are typically accessed through boreholes formed by rotating and advancing a drill string, which may be many thousands of feet long, into the ground. Existing dual string drilling systems include an inner string or pipe rotating within an outer string or pipe. At the bottom of the borehole, a drill bit is attached to the inner and/or outer string and is rotated and advanced into the ground to form the borehole. A drilling fluid is pumped into and out of the borehole to lubricate the drill bit and remove cuttings.
Existing dual string drilling systems suffer from several drawbacks. For example, the cuttings are typically returned through an annulus between the inner string and the outer string. The abrasive return fluid, in contact with the high-speed inner string, causes significant wear, especially when large cuttings get trapped between the high speed driveline and the outer string. The wear can increase costs by shortening the life of the drill strings, or cause drill string failure.
Dual string drilling systems may also exhibit drill string vibration, especially when the drill strings are thousands of feet long. Unexpected and damaging vibrations have been observed in drilling systems even when rotating at relatively low speed (e.g., about 150 RPM).
Depending on the particulars of the drilling system, the drilling system may also experience borehole deflection, especially when hard materials or layers in the earth are encountered. For example, when drilling at an angle through layers of varying hardness (e.g., bedding planes), the drill may tend to follow a softer material, which can cause mild to severe borehole deflection. Since the objective of the drilling process is generally to reach a particular target (e.g., an oil or natural gas deposit), deflection of the borehole can be a critical problem.
Another concern with existing drilling systems is that the drill strings may become disconnected (e.g., spin-off) during drilling. For example, in a high speed dual string system, if both strings have the same thread direction (e.g., right-handed thread) and the faster-rotating inner string catches on the outer string (e.g., due to a cutting chip becoming jammed between the two strings), torque will be transmitted to the outer string from the inner string. The torque transmission may unscrew or spin-off one of the drill string joints on the outer string above the jamming location. At best, time and money will be lost retrieving the disconnected drill string. At worst, both drill strings are lost and the hole must be abandoned.
Needs exist, therefore, for improved drilling systems that minimize wear, reduce vibrations, provide better direction control when drilling through layers of varying hardness, and reduce the likelihood of spin-off.